In a large skillet, heat a splash of olive oil over medium heat. Add the diced red onion and bell pepper, cooking for about 5 minutes until they begin to soften. Add the minced garlic and sauté for an additional minute until fragrant.
In a mixing bowl, combine the sliced zucchinis with the sautéed onion, bell pepper, marinara sauce, Italian seasoning, and a pinch of salt and pepper. Stir gently until the zucchini slices are well coated.
In a 9x13 inch baking dish, evenly spread half of the zucchini mixture across the bottom.
Sprinkle half of the shredded mozzarella cheese over the first zucchini layer.
Top with the remaining zucchini mixture, then sprinkle the rest of the mozzarella cheese. Finish with a sprinkle of grated Parmesan and the sliced black olives. Add red pepper flakes if desired.
Cover the baking dish tightly with aluminum foil and place it in the oven. Bake for 30 minutes.
After 30 minutes, carefully remove the foil and continue baking for an additional 15 minutes.
Once done, take the casserole out of the oven and let it cool for 5 minutes. Then slice into squares or wedges.
If desired, add a sprinkle of fresh basil leaves on top before serving.
Notes
Serve warm and garnish with fresh basil for added flavor.